Logic to compute global stats frame by frame.

We extend the AccumulatedAverage developed earlier, and use a new track
feature analyzer that depends on it, and collect the values it
generated, and arrange them on a frame by frame and per track basis.
We store everything on a ggeneric field of the extended feature model.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>